animals Article Population Density, Size Structure, and Reproductive Cycle of the Comestible Sea Urchin Sphaerechinus granularis (Echinodermata: Echinoidea) in the Pagasitikos Gulf (Aegean Sea) Dimitrios Vafidis 1,*, Chryssanthi Antoniadou 2 and Vassiliki Ioannidi 1 1 Department of Ichthyology and Aquatic Environment, School of Agricultural Sciences, University of Thessaly, 38446 Volos, Greece; [email protected] 2 Department of Zoology, School of Biology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece; [email protected] * Correspondence: dvafi[email protected]; Tel.: +30-241-093-133 Received: 3 August 2020; Accepted: 21 August 2020; Published: 26 August 2020 Simple Summary: Regular urchins are dominant grazers in the shallow sublittoral seabed. Several species are edible and commercially harvested, among which Sphaerechinus granularis is of increasing commercial importance due to the depletion of the common urchin Paracentrotus lividus. As there are very few studies on the biology of the species, the present work examines the population density, size structure, and reproduction of S. granularis in the Aegean Sea (eastern Mediterranean). Population density, in-situ estimated along transects, conforms to previous reports for the Aegean Sea. Size-structure, estimated from 20 randomly collected specimens at monthly base, showed one mode at 65–70 mm or 70–75 mm, according to the sampling location. Sea urchins had larger dimensions in the sheltered site. The monthly variation of the gonad-somatic index and the histological analyses of the gonads, estimated from the same 20 randomly collected specimens (per month and station), showed that the species reproduces once each year, in spring. The results of the present study provides baseline knowledge on the biology of S. granularis and are important for the viable management of its developing fishery. Abstract: Sphaerechinus granularis is a common grazer that lives in various sublittoral habitats, displaying typical covering behavior; i.e., putts shell-fragments, pebbles, and algae on its test. It is an edible species of increasing commercial importance due to the depletion of the common urchin’s, Paracentrotus lividus, stocks. Its biology, however, is not adequately studied over its distributional range. The present study examines population density, size structure, and reproductive biology of S. granularis in the Aegean Sea. Samplings were made with SCUBA-diving (8–10 m) and included: (i) visual census along transects to estimate density, and (ii) random collection of specimens at monthly intervals to assess biometry and gametogenesis. Population density had moderate values that almost doubled when inputted to Distance software. S. granularis had larger dimensions in the sheltered site; size-structures were unimodal (65–70 mm and 70–75 mm, in exposed and sheltered site, respectively). An annual reproductive cycle was evident, according to GSI and gonads’ histology, with a clear spawning peak in early spring. This pattern conforms to previous reports from the Atlantic, but precedes those from the Mediterranean (reproduction in summer). The provided baseline knowledge on the biology of S. granularis is important for the viable management of its developing fishery. Keywords: population abundance; biometry; gonad-somatic index; histology; sea urchin; Sphaerechinus; Aegean Sea Animals 2020, 10, 1506; doi:10.3390/ani10091506 www.mdpi.com/journal/animals Animals 2020, 10, x 2 of 12 1. Introduction The blunt or purple sea urchin, Sphaerechinus granularis (Lamarck 1816), is an edible species distributed in the NE Atlantic and the Mediterranean Sea [1]. The species lives in a variety of habitat Animalstypes2020 [2], 10in, 1506both hard [3] and soft bottoms covered by seagrass meadows [4] or on maërl beds [5]. Its2 of 12 bathymetric distribution expands from the intertidal down to 130 m depth [6], but it is most usually found in the shallow sublittoral (5 to 15 m) where it forms locally dense populations [7]. As other sea 1. Introductionurchins, it manifests a cryptic behavior (Figure 1) by occupying crevices and by camouflaging, as it coversThe bluntits test orwith purple shell fragments, sea urchin, pebbles,Sphaerechinus and algae granularis [8]. (Lamarck 1816), is an edible species The species is harvested for its gonads in France, Spain and in few other countries, since the distributed in the NE Atlantic and the Mediterranean Sea [1]. The species lives in a variety of habitat beginning of 1980s, although it is less appreciated than Paracentrotus lividus (Lamarck, 1816), the types [2] in both hard [3] and soft bottoms covered by seagrass meadows [4] or on maërl beds [5]. target species in sea urchins fishery [5]. In recent years, however, the depletion of P. lividus natural Its bathymetric distribution expands from the intertidal down to 130 m depth [6], but it is most usually stocks is shifting sea urchin fishery to alternative species such as S. granularis, which is a fast-growing, foundlarge-sized in the shallow urchin that sublittoral contains (5 voluminous to 15 m) where gonads it forms [5]. In locally the Hellenic dense populationsSeas, S. granularis [7]. As is locally other sea urchins,consumed, it manifests but it is a not cryptic officially behavior harvested, (Figure and1) byso, occupyingthe minimum crevices landing and size by of camouflaging, 7 cm [9] is not as it coversapplicable its test [7]. with Unfortunately, shell fragments, fisheries pebbles, data are and completely algae [8]. lacking for the region. FigureFigure 1. Covering1. Covering behavior behavior of of the the sea sea urchinurchin SphaerechinusSphaerechinus granularis granularis: :underwater underwater photography photography of a of a specimenspecimen in in station station 2, 2, S2 S2 (photo (photo from from Dr. Dr. Alexios Alexios Lolas).Lolas). TheDespite species the is increasing harvested importance for its gonads of S. ingranularis France, as Spain a fishery and inresource, few other its biology countries, is scarcely sincethe beginningstudied.of More 1980s, explicitly, although the it existing is less appreciatedinformation thanis limitedParacentrotus to the reproductive lividus (Lamarck, biology 1816),[1,5], and the the target specieslife strategy in sea urchinsof the species fishery in [relation5]. In recent to food years, availability however, [10] the in the depletion western of MediterraneanP. lividus natural and stocksthe is shiftingNE Atlantic sea (Brittany). urchin fishery In Brittany, to alternative the species species has a rather such brief as S. breedi granularisng period, which that begins is a fast-growing, in spring (April or May) and extends until early summer (June), in dependence to the annual thermal regime large-sized urchin that contains voluminous gonads [5]. In the Hellenic Seas, S. granularis is locally [5]. Seawater temperature affects mainly the onset of spawning, which exhibits inter-annual consumed, but it is not officially harvested, and so, the minimum landing size of 7 cm [9] is not variability in relation to this environmental factor [9]. In Mediterranean, the breeding period begins applicable [7]. Unfortunately, fisheries data are completely lacking for the region. in summer [1,11] but may extend until November [1]. It is widely accepted that the triggering factor initiatingDespite spawning the increasing is the rapid importance rise in temperature of S. granularis thatas usually a fishery occurs resource, in spring its in biology temperate is scarcely seas studied.[5,9,11], More although explicitly, differences the existing in food information quality are al isso limited involved to the[1]. reproductiveFor the Aegean biology Sea, there [1,5 ],are and no the liferelevant strategy studies of the other species than in a relation few reports to food on its availability distribution [10 [12]] in and the po westernpulation Mediterranean ecology [7]. and the NE AtlanticWithin (Brittany). this context, In Brittany, the present the work species aims has to a ratherstudy the brief population breeding density, periodthat the beginssize structure in spring (Apriland orthe May) reproductive and extends biology until of early S. granularis summer in (June), the central in dependence part of the Aegean to the annual Sea (Pagasitikos thermal regime Gulf). [5]. Seawater temperature affects mainly the onset of spawning, which exhibits inter-annual variability in relation2. Materials to this environmentaland Methods factor [9]. In Mediterranean, the breeding period begins in summer [1,11] but may extend until November [1]. It is widely accepted that the triggering factor initiating spawning is the2.1. rapidStudy Area rise inand temperature Field Sampling that usually occurs in spring in temperate seas [5,9,11], although differencesThe inpresent food qualitystudy was are carried also involved out in Pagasitikos [1]. For the Gulf, Aegean central Sea, Aegean there areSea, no which relevant is an studies enclosed, other thanshallow a few water, reports meso-oligotrophic on its distribution gulf [12 [13].] and Two population stations were ecology sampled: [7]. S1 (39°18′19.3″ N, 22°56′35.3″ E)Within and S2 this (39°18 context,′25.1″ the N, present 23°5′50.9 work″ E), aims located to study at the the north population and northeast density, theparts size ofstructure the Gulf, and therespectively reproductive (Figure biology 2). The of S. sea granularis bottom wasin the sandy central in both part stations, of the Aegean either unvegetated Sea (Pagasitikos or covered Gulf). by moderately